1. Field of the Invention
This invention relates generally to beam index color television receivers and, more particularly, is directed to apparatus for controlling the horizontal beam scanning rate in a beam index color television receiver.
2. Description of the Prior Art
Beam index color television receivers are well known in the art, and usually include a cathode-ray tube, or picture tube, having an electron gun which emits a single electron beam and a phosphor screen in which red, green and blue color phosphor stripes are repeatedly arranged in the horizontal beam scanning direction. The beam index picture tube also has a plurality of index phosphor stripes arranged across the phosphor screen in the horizontal beam scanning direction in a known relationship to the spacing of the color phosphor stripes. When the single electron beam horizontally scans the screen, a photodetector generates an index signal in response to light emitted each time an index stripe is struck by the electron beam. This index signal is supplied to, for example, a phase-locked loop circuit (PLL circuit) for achieving color registration by controlling the color switching circuit which determines when the three primary color signals respectively modulate the intensity of the electron beam so that, at any moment, the intensity of the electron beam is modulated by the primary color signal whose corresponding color phosphor stripe is then being scanned by the beam. Thus, the electron beam is density-modulated by the red primary color signal when the electron beam scans the red color phosphor strips, by the green primary color signal when the electron beam scans the green color phosphor strips, and by the blue primary color signal when the electron beam scans the blue color phosphor stripes, respectively.
The frequency of the index signal is inversely proportional to the pitch of the index phosphor stripes and is proportional to the electron beam scanning rate in the horizontal direction. Therefore, if the horizontal scanning rate is varied, the frequency of the index signal fluctuates.
A time lag occurs during the time from detection of the index signal by the photodector to color switching. As a result, if the horizontal scanning rate varies so as to result in fluctuation in the frequency of index the signal, as previously mentioned, a consequent timing deviation results in the color switching which, in turn, results in color misregistration. Deviations in the horizontal scanning rate are also disadvantageous in beam index color television receivers because they make it more difficult for the PLL circuit to correctly follow and lock onto the index signal, as is necessary for proper color registration.
Accordingly, in a beam index color television receiver, it is desirable to provide for correction of the horizontal scanning rate to reduce any distortion in the horizontal deflection of the electron beam.
A horizontal scanning rate correction apparatus for reducing distortion in the horizontal deflection of the electron beam, that is, which provides for correction of the horizontal scanning rate, is disclosed in U.S. patent application Ser. No. 06/100,820, filed Dec. 6, 1979, and having a common assignee herewith. In such application, information regarding the horizontal scanning rate which has previously been written into a memory is used, during deflection of the electron beam, to drive a horizontal deflection correcting device, such as a horizontal deflection correcting coil, to further deflect an electron beam, thus so as to correct the horizontal scanning rate.
However, even with the above apparatus, a high voltage fed to the cathode ray tube fluctuates in response to changes in the cathode ray tube current which causes the size of the raster on the picture screen, to also change. This causes the electron beam deflection speed to change and, as a result, there is a deviation in the horizontal deflection linearity.